1. Field of the Invention
This invention relates to a safety device provided for a pressure vessel, such as a syrup tank for use in a vending machine.
2. Prior Art
Some vending machines include a syrup tank for supplying a syrup (condensed liquid) under pressure to a cooking block thereof. The syrup tank of this kind is adapted to be pressurized by carbon dioxide gas or nitrogen gas regulated approximately to 4 kg/cm.sup.2 by a gas regulator, for supplying the condensed liquid held therein to the cooking system. Although such syrup tanks do not fall within a legal category of the pressure vessel, they are provided with a safety device, taking the safety into consideration.
FIG. 1A and FIG. 1B show a conventional safety device provided for a syrup tank. This safety device is comprised of a easily-breakable portion 62 formed in a cover 61 of the tank. The cover 61 is made of stainless steel, and the easily-breakable portion 62 thereof is formed by pressing central portions of the cover into two arcuate projections, and then cutting off tops of these projections. That is, the easily-breakable portion 62 is shaped into thinner portions by cutting compared with the remaining part of the cover, whereby these portions are preferentially permitted to be broken when pressure within the syrup tank exceeds a predetermined burst pressure at which these portions are to be broken, thus functioning as the safely device. For example, a cover 61 having a thickness of 1.5 mm is formed with an easily-breakable portion 62 having a thickness below 0.1 mm, which satisfies a specification of a burst pressure level of 21 kg/cm.sup.2 to 35 kg/cm.sup.2.
The working pressure of such a syrup tank, however is 4 kg/cm.sup.2, so that a high burst pressure of 21 kg/cm.sup.2 to 35 kg/cm.sup.2 of the safety device may cause excessively high pressure applied to the syrup tank or pressure piping, if there is failure in the gas regulator or other related parts and devices. This problem can be solved by setting the burst pressure of the safety device to a lowered level of 10 kg/cm.sup.2 or its vicinity. However, this requires precision processing of a stainless steel plate to a thickness of an order of 0.01 mm in forming the easily-breakable portion 62 of such a safety device, which is conventionally formed by pressing and cutting to a thickness of 0.1 mm. It is expected that this precision processing by pressing and cutting requires an extremely high craftsmanship or skill in manual operations, and further it is practically impossible to effect such processing, when taking the composition of stainless steel (homogeneity of its metallographic structure) into consideration. That is, the construction of the conventional safety device suffers from a limitation in respect of processing techniques, in delimiting the burst pressure applied to the easily-breakable portion 62 to a low level of 10 kg/cm.sup.2 or its vicinity.
If the cover 61 per se is formed of stainless steel having a small thickness, it is relatively easy to form recesses reduced in thickness which will be broken at a desired burst pressure. However, the reduced thickness of the cover 61 causes a problem of insufficient strength thereof.